1. Field of the Invention
One or more embodiments of the present invention relate to an electrode for a fuel cell and a fuel cell employing the same.
2. Description of the Related Art
Fuel cells, which use a polymer electrolyte membrane as an electrolyte, operate at a relatively low temperature and can be small in size. Thus, fuel cells may be used as power sources in electric vehicles or distributed generation systems for homes. As a polymer electrolyte membrane used in polymer electrolyte fuel cells, a perfluorocarbonsulfonic acid-based polymer membrane such as NAFION (registered trademark) has been used.
However, such polymer electrolyte membranes typically need water to provide proton conduction abilities, and thus, the polymer electrolyte membranes typically need to be humidified. In addition, to enhance cell system efficiencies, it may be necessary to operate polymer electrolyte membranes at a high temperature of at least 100° C. However, the moisture in polymer electrolyte membranes may evaporate at this temperature, and the polymer electrolyte membranes may not function properly as a solid electrolyte.
To address those problems in the art, non-humidified electrolyte membranes that can operate at a high temperature of at least 100° C. under nonhumidified conditions have been developed. For example, U.S. Pat. No. 5,525,436 discloses polybenzimidazole doped with a phosphoric acid, and the like as a material constituting non-humidified electrolyte membranes.
In addition, in cells that operate at a low temperature, such as cells using a perfluorocarbonsulfonic acid-based polymer membrane, to prevent gas diffusion in electrodes due to water (formation water) that is produced as electricity is generated in an electrode, particularly a cathode, electrodes using polytetrafluoroethylene (PTFE) as a waterproof agent to have hydrophobic properties have been widely used (see, for example, Japanese Patent Laid-Open Publication No. hei 05-283082).
In addition, phosphoric acid type fuel cells operating at a high temperature of 150 to 200° C. use a liquid phosphoric acid as an electrolyte. However, a large amount of the liquid phosphoric acid is present in electrodes, which interferes with gas diffusion. Therefore, an electrode catalyst layer that is formed by adding polytetrafluoroethylene (PTFE) as a waterproof agent to an electrode catalyst, and which can prevent fine pores in electrodes from being clogged by a phosphoric acid, has been used.
In addition, in fuel cells using a polybenzimidazole (PBI) electrolyte membrane, which retains phosphoric acid as a nonhumidified electrolyte at a high temperature, to reduce contact between electrodes and the electrolyte membrane, a method of impregnating electrodes with a liquid phosphoric acid has been tried and a method of increasing a loading amount of metal catalysts has been tried. However, such fuel cells have not exhibited improved properties, and thus there is a need for improvement.
In addition, when air is supplied to a cathode when a solid polymer electrolyte doped with phosphoric acid is used, the fuel cell requires an aging time of about 1 week even if the composition of the cathode is optimized. By supplying oxygen to the cathode instead of air, performances of the cathode can be improved and the aging time can also be reduced. However, the need to supply of oxygen to the cathode is an obstacle in realizing widespread use of the cathode. In addition, a polymer electrolyte membrane formed of PBI typically does not have satisfactory mechanical properties and chemical stability at a high temperature and capability of retaining a phosphoric acid.